But, I am a personal trainer after all and I should be leading by example right. Thank God for air conditioned gym's though...
A 20 minute workout can be plenty if you keep the intensity up and the rest time between exercises and sets down to a minimum, usually just enough to get your breath back.
Here it is and I have shown the weights I used for illustration purposes and you can see how I have increased the weight on each set
1. Dumbbell Chest Press - 10/12.5/15 kg - 12 Reps
2. Reverse Grip Lat Pulldown - 33/40/40 kg - 12 Reps
3. Dumbbell Deadlifts - 10/12.5/15 kg - 15 Reps
1-3 is done as a small circuit (or Tri-Set) with no rest in between but taking approximately 30 seconds rest between sets/circuits. Repeat for 3 sets in total.
4. One Arm Bridge - 1 Minute (15 seconds per arm and swap x4)
5. Prone Hold (on elbows) - 1 Minute
6. Pushup Bridge - 1 Minute (same as 1 but with both arms for the minute)
Repeat as for 1-3 but only ONCE
7. 1 Arm Alternating Bicep Curls - 10/12.5/12.5 kg - 12/10/10 Reps (each arm)
8. Bench Tricep Dips - 15/15/15 Reps
Repeat as for 1-3 (2 exercises back to back is referred to as a 'Superset')
That's it! Obviously this could be a number of different exercises and the weights will vary from person to person. But what is important here is the short space of time allowed and how much you can get done if you keep the rest period down which really elevates the heart rate.
If you need any further explanations of any of the exercises let me know and I will get onto it.